1. Field of the Invention
The present invention relates to a device for digitally determining and safeguarding the thermal image of an apparatus or a load, more particularly applicable to thermal protection relays using a microprocessor for elaborating the thermal image of a motor.
2. Description of the Prior Art
Some electronic thermal protection relays for motors have a microprocessor which serves more particularly for simulating the thermal state of the motor from the measurement of the currents thereof; this simulation is made in accordance with well known heating and cooling exponential curves.
it is desirable for the protection relay to continue simulation of cooling for example for several tens of minutes after a power-cut in the supply to the motor, this power-cut occurring either under the effect of the protection relay whose thermal image has exceeded and admissible threshold, or under the effect of a trip-out or a cut in the power supply to the motor and the relay. In this latter case, continuing the simulation of cooling involves maintenance of the power supply for a part at least of the electronic circuits of the relay after switching off so as to have, during the subsequent starting up again of the motor, an acceptable thermal image thereof. The thermal modelization may be continued by means of the microprocessor, by supplying it with a battery or a buffer capacity; this solution provides however expensive and too restrictive as to the choice of the technology and of the type of microprocessor used.
It is then important to design a reduced consumption device capable of being added to the microprocessor and to be supplied should the power supply to the relay fail by a relatively low buffer capacity for continuing the elaboration of the thermal state during the duration of the shut down.
The Pat. No. DE-3 137 496 has proposed storing in the capacity of an RC circuit, at the time of the voltage trip-out caused by the relay, the thermal image elaborated by the microprocessor, then letting the capacity discharge into the resistor of the RC whose time constant allows the simulation of the characteristic cooling exponential of the motor to be continued; on switching on again, the microprocessor reads the analog voltage value at the terminals of the RC by means of an analog-digital converter and continues the simulation.
Such a device has the drawback of requiring an analog member and a correlative analog-digital conversion, even digital-analog conversion if it is desired to charge the capacity with a voltage which is a function of the thermal image.
The applicant starts from the discovery that simulation of the cooling of the motor during a voltage trip-out may be simplified by substituting for the cooling exponential, at least one straight line segment whose slope is chosen so as to be adapted to the "hot" part of the exponential.